31 July 2017 (CU Boulder Today) – Even if humans could instantly turn off all emissions of greenhouse gases, Earth would continue to heat up about two more degrees Fahrenheit by the turn of the century, according to a sophisticated new analysis published today in the journal Nature Climate Change.

If current emission rates continue for 15 years, the research shows, odds are good that the planet will see nearly three degrees Fahrenheit (1.5 Celsius) of warming by then.

“This ‘committed warming’ is critical to understand because it can tell us and policymakers how long we have, at current emission rates, before the planet will warm to certain thresholds,” said Robert Pincus, a scientist with the Cooperative Institute for Research in Environmental Sciences (CIRES), a partnership of the University of Colorado Boulder and NOAA. “The window of opportunity on a 1.5-degree [C] target is closing.”

During United Nations meetings in Paris last year, 195 countries including the United States signed an agreement to keep global temperature rise less than 3.5 degrees F (2 C) above pre-industrial levels, and pursue efforts that would limit it further, to less than 3 degrees Fahrenheit (1.5 C) by 2100.

The new assessment, co-authored by Pincus and Thorsten Mauritsen of the Max Planck Institute for Meteorology, is unique in that it does not rely on computer model simulations, but rather on observations of the climate system to calculate Earth’s climate commitment. Their work accounts for the capacity of oceans to absorb carbon, detailed data on the planet’s energy imbalance, the climate-relevant behavior of fine particles in the atmosphere and other factors.

Among Pincus and Mauritsen’s findings:

Even if all fossil fuel emissions stopped in 2017, warming by 2100 is very likely to reach about 2.3 F (range: 1.6-4.1) or 1.3 degrees C (range: 0.9-2.3).

Oceans could reduce that figure a bit. Carbon naturally captured and stored in the deep ocean could cut committed warming by 0.4 degrees F (0.2 C).

There is some risk that warming this century cannot be kept to 1.5 degrees C beyond pre-industrial temperatures. In fact, there is a 13 percent chance we are already committed to 1.5-C warming by 2100.

“Our estimates are based on things that have already happened, things we can observe, and they point to the part of future warming that is already committed to by past emissions,” said Mauritsen. "Future carbon dioxide emissions will then add extra warming on top of that commitment.”

The research was funded by the Max-Planck-Gesellschaft, the U.S. Department of Energy and the National Science Foundation.

31 July 2017 (Max Planck Institute for Meteorology ) – Concerted efforts around the world are now being made to limit global warming to less than 1.5 to 2 degrees Celsius. But how far are we from passing these thresholds? Climate models have been used to estimate this emission headroom, but a new study by Thorsten Mauritsen from the Max Planck Institute for Meteorology (MPI-M) and Robert Pincus from University of Colorado published in Nature Climate Change instead uses observations of global warming at the Earth's surface and in the oceans to infer the future warming to which the planet is already committed by past emissions, and find that the window of opportunity on the 1.5 degrees target is closing.

The Earth is about 0.8 degrees Celsius warmer today than at the dawn of industrialization. This warming is due mainly to human emissions of the greenhouse gas carbon dioxide through burning of fossil coal, oil and gas. Because the oceans have such an enormous capacity to absorb heat they are out of balance with even current concentrations of CO2 and balance would be reached only after hundreds or thousands of years. Because CO2 is removed from the atmosphere at the same long time scales, human influence on Earth's climate will continue long after emissions from fossil fuel burning eventually stop. The influence of past emissions on future temperature is often called "committed warming."

Carbon dioxide is not the only influence on future temperature changes. Some amount of warming is masked by polluting aerosol particles also associated with fossil fuel burning. These aerosols reflect sunlight back to space, cooling the planet's climate. But in contrast to CO2 their lifetime in the atmosphere is very short, so that a short burst of rapid warming is expected when emissions stop. Working in the opposite direction are various short-lived greenhouse gas pollutants such as methane, nitrogen oxides and carbon monoxide. All these gases contribute to warming but have short lifetimes -- less than a decade - leading to a somewhat cooler future after emissions are stopped.

After considering these effects the authors estimate that warming at the end of the century would probably reach 1.3 degrees Celsius (0.9-2.3) in the hypothetical case that all fossil fuel emissions would suddenly stop in year 2017. This leaves fairly little headroom until the 1.5 degrees target is surpassed: about 15 years at the current rate of emissions until the risk of exceeding the 1.5 degrees target reaches 50 percent.

Nature might help a bit, though, by removing some of the CO2 from the atmosphere and upper ocean and burying it in the deeper oceans. It is still too early to estimate how strong the removal is using observations, but climate models suggest the effect is important. By accounting for such an effect in their study Mauritsen and Pincus find that committed warming is 1.1 degrees Celsius (0.7-1.8) at the end of the century.

By considering the error bars in each factor the authors are able to put error bars on their estimates. The error bars imply that for example, there is some risk that the 1.5 degrees target is already impractical. This uncertainty will decrease quickly in the coming years as observations improve and the warming signal becomes stronger.

ABSTRACT: Due to the lifetime of CO2, the thermal inertia of the oceans1, 2, and the temporary impacts of short-lived aerosols3, 4, 5 and reactive greenhouse gases6, the Earth’s climate is not equilibrated with anthropogenic forcing. As a result, even if fossil-fuel emissions were to suddenly cease, some level of committed warming is expected due to past emissions as studied previously using climate models6, 7, 8, 9, 10, 11. Here, we provide an observational-based quantification of this committed warming using the instrument record of global-mean warming12, recently improved estimates of Earth’s energy imbalance13, and estimates of radiative forcing from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change14. Compared with pre-industrial levels, we find a committed warming of 1.5 K (0.9–3.6, 5th–95th percentile) at equilibrium, and of 1.3 K (0.9–2.3) within this century. However, when assuming that ocean carbon uptake cancels remnant greenhouse gas-induced warming on centennial timescales, committed warming is reduced to 1.1 K (0.7–1.8). In the latter case there is a 13% risk that committed warming already exceeds the 1.5 K target set in Paris15. Regular updates of these observationally constrained committed warming estimates, although simplistic, can provide transparent guidance as uncertainty regarding transient climate sensitivity inevitably narrows16 and the understanding of the limitations of the framework11, 17, 18, 19, 20, 21 is advanced.